Subtotal nephrectomy in rats, the 5/6ths ablation model, leads to progressive loss of kidney function and chronic renal failure. We postulated that the progressive glomerular lesions were due to an early increase in the turnover of glomerular resident cells. This leads to abnormal glomerular growth, with an increase in the glomerular volume which was detectable using morphometric measurements. We have performed autoradiographic studies, using 3H-thymidine, and found that within two days following subtotal nephrectomy there was an increase in the glomerular cell mitotic index, as well as an increase in the turnover of the cells in the arterial wall. These findings suggest that dysregulation of cell growth is an early event in the development of glomerulosclerosis following reduction in renal mass.